The manufacture of semiconductor devices and integrated circuits involves the blanket and selective deposition and removal of many layers of conductive, insulating and semi-conductive materials on substrates that are usually in the form of silicon wafers. The manufacturing processes typically include the formation of a series of metal interconnect film stacks on a wafer by a plurality of sequential processes performed in a series of processing chambers of one or more multi-process vacuum processing tools. Between the formation of the various stacks of the series, wafers are typically removed from a vacuum processing tool and a photo-resist pattern is applied thereto. The application of the pattern is followed by reactive etching processes that are rendered selective by the pattern. By these processes, troughs and/or holes through insulating layers on the underlying stack are formed, exposing contact areas on underlying conductors that are to be connected to the conductors of devices of subsequently applied overlying layers. Before such layers are applied, the masking layer may be removed.
Following selective etching and, in the case of the first metal layer also following an ion implantation process, patterned wafers are reintroduced into a processing tool where a subsequent stack of conductive layers is applied. The lowermost layer of the new stack to be applied is usually a layer of a reactive elemental metal such as titanium, chromium or tantalum, but may also be a metal nitride, silicide or alloy. One function of this lowermost metal layer is to form a bond or contact with an exposed conductive layer, such as silicon or metal, at the bottom of a contact hole in the underlying insulator. The bond serves to form the initial film portion of a conductive path between the underlying layer and the conductor of a new layer of the new stack.
Before the metallization layer is applied, however, it is usually necessary to clean from the wafer native oxides and other contaminants that characteristically formed on the contacts during prior processes or when the wafer was transferred through atmosphere from tool to tool. Even if such wafers were transferred under vacuum, the vacuum is not perfect so contaminating layers of atoms and molecules usually have formed on the surfaces of the contacts in proportion to the exposure duration. Such contaminating layers would, if not removed, interfere with the application of the metallization layer, usually resulting in degraded conductivity between the contact and the metallization layer.
The standard approach to dealing with the problem of contaminants on a contact surface is to subject the wafer to an inductively coupled plasma (ICP) soft sputter etch step immediately before initiating the metallization process. Such a soft etch step is typically carried out by first transferring the wafer, after placement into a vacuum processing tool in which the new stack is to be applied, into a soft etch chamber. In the soft etch chamber, a plasma is formed of an inert gas, usually argon. Then the plasma ions are electrically accelerated toward the wafer, usually by applying a bias to the wafer. The contaminant materials removed from the contacts by sputtering redistribute through the process chamber or onto the walls of high aspect ratio features, where they do not interfere with the subsequent electrical contact. Such a soft sputter etch is additionally beneficial in that it produces a uniform repeatable surface that facilitates the manufacturable deposition of PVD and CVD films.
The argon soft etch is not an ideal cleaning process since it cleans only by physical removal of contaminants afforded by sputtering. Such sputtering can damage the structure to be cleaned or the underlying device structures, either due to the mechanical sputtering action or through the accumulation of charge. Further, the argon is chemically inert and thus does not react with or chemically reduce the native oxides and other contaminant materials that are to be cleaned from the wafer surface.
Additions of reactive gases to the soft etch plasma have aided in the removal of contaminants from the contacts during soft etch cleaning, but have generally been found undesirable in other ways, particularly since these gases tend to migrate out of the process area and contaminate other portions of the process tool. Further, reactive components such as hydrogen can damage collateral device structures since hydrogen and other commonly used reactive components readily diffuse through the wafer.
In addition, freshly soft etched surfaces can be recontaminated in a processing tool by gases such as water vapor and oxygen from normal outgassing and from gases originating from CVD process modules. Further, the need for separate etch and deposition modules adds to the product cost and to the size of the processing equipment.
Accordingly, there is a need for a more effective and less costly process for preventing oxides and other contaminants from interfering with the metallization of surfaces at which contacts on the lowermost layer of a stack or other interconnects are to be formed, for example, on intra-stack layers that are otherwise prone to oxidation or contamination with water vapor or other material prior to the metallization of the such surfaces.